Surface ocean currents, in the upper 1000 meters of
the ocean, are driven by winds and are strongly influenced by change in
density between layers of water. These ocean currents tend to follow the
winds in the lower atmosphere, except where they are diverted by continents.
Some wind-driven currents can even go upwind. An example is the North
Equatorial Countercurrent.

Thermohaline Circulation

Because temperature and salinity determine density,
deep-water circulation is called thermohaline circulation.

Higher density water sinks into the deep ocean.

Mixing brings cold water up to the surface.

The thermohaline circulation is actually driven by mixing.

Ekman Currents

Wind force, friction, and Coriolis are three forces
that act on water at the sea surface.

When the three forces balance, the current goes 45 degrees
to the right looking downwind in the Northern Hemisphere, This is the
Ekman current.

Current turns with depth. Each layer goes at an angle
to the layer above it.

Ekman transport of surface layer water tends to produce
sloping sea surfaces by "piling up" the water in some locations
and "removing" surface water from others. This causes presssure
gradients inside the ocean.

Inside the ocean the only important forces are pressure
gradients and Coriolis force due to ocean currents.When the two forces
balance, the current is a geostrophic current.

Upwelling and Downwelling

Upwelling occurs where Ekman transport moves surface
waters away from a coast or away from nearby areas (a divergence zone).
As the winds carry the surface waters away from a coast, cooler, often
more nutrient-rich, water from below replaces the surface waters.

A "conveyor belt" model of this connection
suggests that warm water from the Pacific and Indian Oceans is exported
to the Atlantic Ocean along the surface, where it cools and sinks off
Norway and Greenland, returning at depth back to the Pacific and Indian
Oceans. As heat is lost to the atmosphere in the north, the colder water
sinks below the warmer surface layer and migrates through the depths of
the global oceans.

The entire circuit takes as long as 1,000 years to complete.

The heat lost in the far North Atlantic warms Europe.
If the conveypor belt circulation weakens or stops, Europe and the Northern
Hemisphere could be plunged into a new ice age.